It is understood that many or certain internal combustion engines for vehicles are unable to generate any torque during standstill, so that the engine is also unable to start up by itself. Therefore, generally an induction cycle and a compression cycle must be triggered to start the internal combustion engine. An auxiliary unit, colloquially called a starter or starter motor, usually fulfills this function. It is an electric motor operated by a 12V battery in the vehicle.
Printed patent publication EP 115 18 92 A2 refers to recharging the 12V battery from a high-voltage battery prior to starting the internal combustion engine, if the output voltage of the 12V battery is too low. Printed patent publication U.S. Published Patent Application No. 2001 041 952 discusses adjusting the power drawn from the high-voltage battery based on a comparison of the output voltage of the 12V battery to various reference voltages. Printed publications JP 2001 320 807 A, U.S. Pat. No. 6,522,960 B2 and JP 33 81708 B2, refer to charging the 12V battery upon recognition its output voltage is too low, and at the same time, to check whether the instantaneous output voltage of the 12V battery is sufficient to immediately start the internal combustion engine.
This procedure is very proficient for the operation of an axle split hybrid or other hybrid vehicle in which the auxiliary unit for starting the internal combustion engine is operated from a 12V battery. For example, if the steady-state voltage of the 12V battery is too low because of too long a time parked, or because of a high no-load current, and therefore too great a discharge, then the battery is first recharged before starting an internal combustion engine. A presumably failing attempt to start the internal combustion engine is thereby avoided, since it would probably only lead to an even greater rundown of the battery.
However, it often turns out that attempts to start the internal combustion engine nevertheless prove unsuccessful, even though the system described above detects a sufficiently high output voltage of the 12V battery for the operation of the auxiliary unit to start the internal combustion engine.